1. Field of the Invention
This invention relates to the field of digital imaging and more particularly to the field of watermarking digital images.
2. Related Art
Digital imagery provides a means for many and/or widely scattered users to have access to a single object, such as a manuscript in a library, an object in a museum, a magazine article, or anything else which can be photographed or scanned. A digital image is a two-dimensional array of picture elements (pixels), each of which describes the color of the object at one point. Digital images may be conveniently stored in a digital computer, transmitted over communications lines, and reproduced at a remote location.
One of the significant deterrents to making large libraries of digital images publicly available is the concern, by the owners of the image content, about misappropriation of their images. In many cases, the owner of the medium from which the image was digitized earns revenue from some uses of the images. In this case the owner typically will wish to prevent the images from being copied and then used, royalty-free. An example of this might be a publisher who makes images available within the context of a digital book or magazine, but who also wishes to prevent the unauthorized copying and use of these images by other publishers. Another example of this might be that of an owner of an art collection who wishes to sell images of art objects in the collection for multimedia presentations, but does not want these images used for publishing books of art.
In other cases, the owner of the media may wish to prevent certain uses for other reasons. A national library, for instance, might be willing to make reproductions of various works available for study, yet unwilling to make them available in any form that might be used to advertise a product, such as pornography, that would be embarrassing to that institution.
Hence, the general problem is to devise techniques that produce images that are totally acceptable for some uses, and yet unacceptable for other uses. Frequently, it is desired to produce images that are entirely acceptable for inspection or study, yet unacceptable for publication. One method of accomplishing this is known as "watermarking".
A simple watermarking method is demonstrated in Jim Pickerell and Andrew Child, Marketing Photography in the Digital Environment, 1994. "Image Watermarking for Photoshop", an additional sheet available from the same company, gives instructions for applying a watermark using Photoshop. Pickerell and Child use the technique for protecting electronic catalogs of photographs and "clip art".
While watermarking is an effective way for copyright and media owners to control the use of their images, conventional watermarking processes can alter the chromaticities of the original image at points where the watermark appears. This effect may be undesirable from the perspective of both the viewer and the owner of the original image.